The present invention relates to a scanning method used by a laser radar for scanning a desired region with a laser beam to detect a target, and a laser radar for carrying out the scanning method.
In a conventional laser radar, as described in a Japanese patent application JP-A No. 57-125,371, a laser beam performs a raster scanning.
In a raster scan, as shown in FIG. 13, a laser beam 5 makes a linear scanning in a predetermined scanning region 1 in each of X- and Y-directions.
In a laser radar, it is required to make small the divergence angle of a laser beam, and thus an optical system for shaping and transmitting the laser beam is obliged to become large in size and weight. The above optical system is driven so that a desired region is scanned with the laser beam. In a case where the laser beam performs a raster scanning operation, a body large inertia is required to move linearly. Thus, there arises a problem that drive means such as X- and Y-scanners become large in size. The divergence angle 1/4 of a laser beam is given by the following equation: EQU tan1/4=1.22.lambda./D (1)
where D indicates the effective diameter of a lens, and .lambda. the wavelength of the laser beam. For example, in order to make the angle 1/4 equal to 10 .mu. rad in a case where the laser beam has a wavelength of 10.6 .mu.m, it is necessary to make the effective diameter of the lens greater than about 1.3 m. When the lens is made of fuzed silica having a density .function. of 2.2 g/cm.sup.3, the weight of the lens is in the order of 600 Kg. A maximum torque T.sub.max necessary for rotating the lens in a time t.sub.F equal to 100 ms through an angle .theta..sub.F equal to 0.1 rad by a bang-bang control, is given by the following equation: ##EQU1## where J indicates the moment of inertia of the lens.
When a disc having a diameter of 1.3 m and a thickness of 0.2 m is used as an approximation of the lens, the moment of inertia J is nearly equal to 60 Kgm.sup.2. Thus, it is known from the equation (2) that the maximum torque T.sub.max is about 2400 Nm. From an equation T=D.multidot.F with respect to a rotational motion (where F indicates a drive force), it is known that the drive unit is required to have a maximum output F.sub.max equal to 1800 N.
In a radar using a microwave, as described in a Japanese patent application JP-A No. 51-85,392, a microwave beam makes a spiral scanning different from a raster scanning. In this case, however, the microwave beam makes the spiral scanning for the purpose of finding a target rapidly, and continues the outward spiral scanning till the target is found. That is, the repetition of the scanning for the same region is not taken into consideration. Further, in some cases, the spiral scanning is mixed with a linear scanning. That is, the above-mentioned problem of a laser radar has not been recognized.